Control system



Nov. 10, 1942. M. A. EDWARDS CONTROL SYSTEM 2 Sheets-Sheet l Filed Sept. 6, A1.941

Inventor Ba A6. .m

L ,M Y b CURRENT WW. n.. P O m A .s H

NOV l0, 1942- M. A. EDWARDS 2,301,688

CONTROL SYSTEM Filed Sept. 6, 1941 2 Sheets-Sheet 2 v Inven-tor: Martin A. Edwards,

` His Attorney.

Patented Nov. 10, 1942 2,301,688 CONTROL SYSTEM Martin A. Edwards,

eral Electric Company,

York

Scotia, N. Yeasslgnor to Gena corporation o! New Application September 6, 1941, Serial No. 409,789 12 Claims. (Cl. 172-239) This invention relates to control systems, more particularly to systems for controlling the operation of electric motors and it has for an object the provision of a. simple, reliable and efficient control system of this characten More specifically, the invention relates to control systems for electric motors in which the "motor drives an inertia,

to repeated reversals which are accomplished by "plugging the motor. ACiperetions of this char- ,actor produce abnormally large peek amature currents and these peek currents impose severe strain both on the apparatus driven by the motor and on the driving connections between the n10- tor end the driven apparatus. Accordingly, a further object of the invention is 'the provision of means for limiting the armature current peuk on reversal to soie velues.

There ere merry industriel applications in which a. motor must be operated on en intermittent and continuously repeated reversing duty cycle 0n such applications, the motor may be started, stopped sud reversed several thousands oi times in e der'. The production of the op paretus driven by such e, motor is determined to a. very considerohle'decree by the speed with which such storting end stopping und reversal operations een be effected uuthout domage 'to the motor or concretos driven thereby. llt is thus e, further object of this invention to provide means for effecting the storting, stopping and revers/el opereticns of an eiectric motor in e minimum tiene together with rhesus for :ivoiri-u ing dangerous current peaks yso that the epperotus drivenby the motor muy be utilized et its maximum efficiency und productivity without domage itself or the driving motorn 'in csr-tying the invention into eil-"ect in one form thereoi7 on electric motor is supplied :from on adjustable voitoge generator which is pron vided with e. seperately excited field winding. This field Winding excited by means of an armature reaction excited clyriemomelectrie machine which is provided with e control cld winding?. In order that the generator shell have a steeply drooping; voit-ampere characteristic, i. e., a characteristic such that the generator armature current remains ,fairly constant at a. maximum value for variations of the voltage throughout the major portion of the range from sero voltage to maximum voltage, the armature reaction excited machine is provided with an auxiliary iield Winding which is excited by the generator current and which opposes the control ileld Winding, together with means for malnload and is subjected' taining this opposing field winding deenergized until the armature current oi the generator exceeds a predetermined value. A plural position reversing type master switch and on adjustable resistor controlled thereby and connected in circuit With the control field winding serve to adjust the maximum values of the generator armature current in accordance with the position of the master switch. For the purpose of providing e maximum permissible acceleration of the driving motor a. capacitor is connected in parallel with the control eld resistor. This capacitor eiiects field forcing of the adjustable voltage supply generator when the motor is accelerated from rest but does not iorce the ileld during a, plugging operation and therefore has no tendency to augment the high current peaks which are present during braking or reversal opere.- tions. 1n order to prevent dangerously high current peaks during plugging and reversing operations, the armature reaction excited dynarnc-electric .machine is provided with on amiliary opposing field winding which is excited. by the voltage oi this dynamo-electric machine in such u, manner es to limit the rote ci change of the voltage oi the machine except when the motor is being accelerated trom rest.

For a, better and more complete imderstoriding oi' the invention, reference should i ovv be hed to the following speciucetion end to the accompanying druwings in which Fieri is e siro ple, diagremmetical illustration of en ment oi the invention, Fie. 2 is e. simule, die grammatical sketch oi e modiflcetiou und 3 is e. chart of characteristic curves which fucile l itete en understundinsr of the invention.

Referring now to the drawings, e ulurelty oi motors lo and Il are connected to be supplied from the adjustable voltage generator l2. The generator l2 is driven means (not shown) such 'for example es un citer noting current Womad rotor induction motor or a. syiualslronousmotor. The swing; motion of on electric shovel involves cyclic repetition ot the starting, reversing and stopping operations o the driving motors. For exemple, u shovel of average size requires about thirty seconds to coinplete its cycle of operation. When the Clipper reaches the top of the benk, the swing motion motor must be accelerated from rest to iulispeed and it must be plugged` end reversed as it arrives over the car which is being loeded and then returned to the stopped. For e. shovel tinuously, i. e., twenty-four hcursper day,

bottom of the bank and which -is operating conthis embcdiby any suitable driven@ swing motion is repeated about 2100 times per day. Since the present invention is intended for the rigorous operating conditions encountered in this type of service, it may be assumed that the motors I0 and II are mechanically coupled to drive the swing motion of an electric shovel, although it will be understood that the invention has other applications such for example, as the hoist motion of a shovel in which extremely severe operating conditions and requirements are also encountered.

The motors iii and II are provided with shunt field windings lila and ila, respectively, which are connected in parallel relationship with each other and in series with field resistors Il and i5 to a source of excitation which is represented by the two supply lines I8.

The generator I2 is a split output generator having two separate output circuits, one for each of the motors i@ and II. The circuit for the motor .lil extends from the brush |211 through series neld winding I2b, motor commutating field winding Iilb, armature of motor I0, series resistor il, commutating held windings I2c, and series field winding ld to the brush I2e. Similarly, the circuit for the motor i| extends from the brush i 2f, through series field winding I2g, motor commutating neld winding I Ib, armature of motor I i commutating eld windings 22h and series eld winding 21' to brush |29'. Generator I2 is also provided with a main separately excited held winding i2c and a self-excited shunt eld winding ii. The self-excited shunt eld winding iEI is connected from the brush I2o to the terminal of the series eld winding |21' which is remote from the brush 27' and is so connected that it acts cumulatively with the main separately-excited eld winding ic. The series eld windings ib, 52d, iig and @2i are connected to be difierential with respect to the separately-excited eld winding i 2k. These series iield windings have much fewer turns than the usual series dinerential eld windings. They cause the generator to hare a slightly drooping characteristic which pron metes stable operation.

Excitation for the main separately excited generator eld winding iZ/'c is supplied from a special armature reaction excited dynamo-electric ma chine it. This dynamo-electric machine is driven at a speed which is substantially constant by any suitable driving means such, for example, as a synchronous motor (not shown). The dynamoelectric-machine l 8 has two sets of brushes. One

set of brushes I8a is connected to an externalY load which in this case is the separately-excited field winding I2k of the. adjustable voltage generator. The other set of brushes I8?) is short circuited. The axis of the flux which is produced by the current flowing in the short circuited armature conductor is referred to as the short circuit axis and the axis which is displaced 90 electrical degrees from the short circuit axis is referred to as the control axis. If it be assumed that the axis of the short circuit brushes 28h coincides with the short circuit flux axis, then the load brushes |8a are on the control axis. The net iiux along the control axis of machine I8 is produced by one or the other of two opposing main control eld windings I8c and I 8d, series compensating field windings I8f and I8g, current limiting field windings Ish and the armature reaction of the loadcurrent.

It is to be noted that the eld winding IBh is connected to be energized by the voltage drop across the resistor I'I.

winding Ish produces a flux proportional to the current flowing in the loop circuit between the generator I2 and the motor I il and it is connected to be differential with respect to the main field Winding IBd or I8c, whichever happens to be energized. The flux along the short circuit axis of the machine I8 is produced by the armature reaction of the short circuit current. This short circuit axis flux generates the vo] tage which appears across the load brushes |811 and the control axis ux produces a voltage which appears across the short circuited brushes lab and causes short circuit current to flow.

The operation of the machine I8 willbe readily understood by considering the operation without the current limit field Winding |871. and the compensating eld windings If and IBg and then subsequently considering the effect of these fields on the operation.

Assuming that voltage is applied to the control field winding I 8c, a current will begin to build Consequently, the field 75 up in this field. Immediately there is generated a voltage in the short circuit path which circulates a large current in the short circuit.

Since the resistance of the short circuit is so low as to be almost negligible, the control field winding I 8c is only required to produce a very' small fiux in order to produce a large current in the short circuit and a correspondingly large armature fiux. Furthermore, since the iiux of the control field winding need only build up to such a low value, and since the resistance and reactance of the short cirouited armature are very low, full load current in the short circuit will be obtained in an exceptionally short time. 'Die armature reaction flux produced by the short circuit current generates a voltage across the load brushes I8a andv causes load current to iiow. This load current will produce an armature flux which would oppose the flux of the control field winding I8c. This would, of course, mean that the flux of the control held winding I8o would have to be large enough to ever-power this control armature reaction flux. However, by using the series eld windings I8f and |89 through which the load current passes, the armature reaction on the control axis can be completely neutralized. Thus, by the use of these series compensating elds, the control eld is only required to produce enough field flux and voltage to overcome the resistance drop in the short circuit, and the time constant or rate of response is, therefore, very fast.

As pointed out in the foregoing, the flux of the field winding I8h opposes the flux of the control field Winding. The control field winding llc is arranged to be connected to the excitation source I 9 by means of a suitable reversing type multi-position speed-governing master switch 2l. when the master switch 20 is moved to one of its right-hand operating positions. The field windingv I8c causes the machine I8 to increase the voltage of generator I2 and the field winding Ilh which is excited by the armature current of the generator causes the machine I 8 to tend to reduce the voltage of the machine I8 and of the generator I2.

When the dynamo-electric machine Il supplies current to the separately excited field winding I2k of the generator I2, the generator will supply current to the motors I0 and II. If the current supplied to the motor Ii! exceeds such a value that the voltage drop across the resistor I| exceeds the drop between the midpoint on the motor field resistor I4 and either of the reversely connected which the voltage drop a winding I8c. Therefore, with a g rectifiers 2| and 22, current will flow in the current-limit field winding Ish in such a direction as to cause the dynamo-electric machine I8 to deliver less current to the separately excited field winding I2k of the generator. The motor fleld resistor I4 provides a bias voltage with respect to cross the resistor I1 in the loop circuit between the generator and the motor I is compared and the strength of the current limit winding I8h depends upon the amount by which the voltage drop acrossvthe resistor I1 exceeds the bias voltage. The amount of current required to ow in the field winding Ilh in order to `maintain equilibrium conditions is dependent upon the current in the control field iven adjustment of the equipment, various values of current in the control field winding I8c establish various values of current fiowing in the generator output circuit. As the motor I0 accelerates and as the voltage oi the generator I2 builds up, additional separately-excited iield current is required to excite the generator andovercome saturation, so that for a given adjustment of the field winding iBh and agiven current in the control field winding I8c a decrease in the current supplied to the motor would cause the generatorvoltage to int crease. Therefore, the voltage of the generator builds up until, the current supplied by the generator attains a low value and the separately excited field current is just enough to maintain that voltage. The current that will be supplied from the generator is just that amount which is sumcient to cause the motorsk iii and II to operate at constant speed.

i Reversal of the excitation of dynamo-electric machine IB i. e., deenergization oi whichever of the field windings Ic or Id is excited and excitation of the previously unexcited eid winding tends to reverse the current flowing in the separately excited heid winding i276 of the generator. This causes the voltage of the generator i2 to den crease so that the motors II) and Ii pump hack to the generator thereby 'to effect deceleration of the motors. 'in order to prevent too rapid a huiid-up of current peaks upon reversal ofthe control i'ieid of the dynamo-electric machine i8, an antimhunt iieid winding Ii is connected across the load circuit brushes ci the machine It and a capacitor is included in this circuit. The anti-hunt iieid winding I8i is so connected that whenever current ows through the antihunt .iield winding and the capacitor 23 as a result oi a change in the `voltage of the load brushes oi the machine i8, the direction of this current will be auch as to oppose the change in voltage. This dampens the reversai of the separately-excited field winding i216 and thereby prevents undesirable peak currents on reversal.

A. resistor 2t is included in the circuit between the brushes of the dynamolectric machine it and the separately-excited iield winding I2Ft. it will he noted that a portion or this resistor 2d is included in series withv the current limit iield winding Ilh. The voltage across this section or resistor has such a polarity as to cause the voltj age applied to the eld winding Ish to he decreased in proportion to the magnitude of the current flowing in the separately excited iield winding |216. This causes liver relatively higher current at higher voltages than would otherwise be the case.

There are times when it is desired that the anti-hunt eld winding Ii shall not oppose any change in the voltage of the machine I8 but shall the generator to deactually assist the change. For example, when the motors I0 and II are to be stopped by moving the master switch 20 to the neutral position when the dipper is returned to the bottom of the bank, it is desirable that the excitation of the separately-excited field winding I2k shall be rapidly reduced so that the voltage of the generator I2 and the speed of the motors I0 and II will also be rapidly reduced. For this purpose a oontactor 28 whose contacts short circuit the capacitor 23 through a resistor 23a when the master switch 20 is in the neutral position, is provided. When the contacts of this contractor are closed, current flows through the anti-hunt eld winding I8i in such a direction that the flux is opposite to the direction of the residual flux. This permits the anti-hunt field |81' to destroy the residual voltage of the dynamo-electric machine I8 and this promotes rapid reduction of the field current of the separately excited field winding I2k of the generator.

Since the field winding I8h of the dynamoelectric machine I8 remains deenergized until the voltage drop across the resistor I1 in the armature circuit of the generator I2 and motor I0 exceeds the bias voltage, the volt-ampere characteristic of the generator will have the usual slightly drooping characteristic such as is represented by the portion a--b of the curve 25 in Fig. 3. In other words, the voltage remains fairly constant for a substantial change in current over this portion of the characteristic. However, after the voltage drop across the resistor Il' has exceeded the bias voltage, the differential field Ih becomes energized. and effective to reduce the voltage rapidly with further increases in the current, as indicated by the portion b-c of the curve 25. In. other words, for this portion of the char# acteristic relatively small changes in the current taire piace for relatively large changes in the voltage.. In other words, the current remains fairly constant over a wide range of generator voltage,

A plurality of resistors 2i, 28, 29 and 30 are provided for varying the strength of the control field of the dynamo-electric machine I8. These resistors are connected in circuit with the control iielti winding itc when the 'master switch 2c is moved to 'its irst right-hand operating posiu tion and are connected. in circuit with the control field winding ifid when the master switch is irioved to its first left-hand operating position.

For the purpose o forcing the excitation of the f dynarnonelectric machine I8 and thereby forcing the excitation of the main separately excited field winding (12k oi the generator' i2 a capacitor 3| is connected in parallel with the resistor 2l' and a similarcapacitor 32 is connected in parallel with the field adjusting resistors 28, 29 and 3u. The forcing capacitors 3i and 32 have a much iarger capacity than the' anti-hunt capacitor 23. Consenuentiy, the anti-hunt capacitor 23 is unable to limit the rate of rise of voltage of the dynamo-electric machine tu resulting from the forcing action of the capacitors 3i and 32, as when the motors it and II are being accelerated from With 'the foregoing understanding of the apparatus and its organization in the completed system, the operation of the system itself will readily be understood from the following detailed description.. It may be assumed that the dipper has heer; brought to the top of the bank by the v hoist motion of the shovel and that it is desired the rightto a position over to swing the dipper to a loading car. At this instant the control apparatus for the swing motion will be in the condition in which it is illustrated in the drawings. This operation is initiated by moving the master switch 20 to the right.

In the first position of the master switch, the cam actuated contacts 20a and 20D are closed. The closing o contacts 20h completes an energizing circuit for the operating coils of contactors 26 and 33. rihis circuit is readily traced from the positive side 'of the excitation source I8 through conductor 34 and the operating coils of contactors 26 and 33 in parallel and thence through conductor 35 and contacts 20h to the negative side of the excitation source I 9. Contactors 26 and 33 open their contacts in response to the energization of their operating coils. Contactor 33 in opening its contacts interrupts the short circuit around the rectiers 2i and 22 so that a bias voltage to be derived from the voltage drop across the resistor I1 can be introduced into the circuit of the field winding I8h of dynamoelectric machine l 8.

Contactor 28 in opening its contacts interrupts the short circuit about the anti-hunt capacitor 23 thereby rendering it eiective.

Contacts 23a ci the master switch in closing complete an energizing circuit for the main control ield winding lac of the dynamo-electric machine I. 'Ihis circuit is traced from the positive side of the excitation source I9 through resistors 21, 28, 29 and 353 in series, normally closed contacts of Contacter 35, control field winding I8c and thence through contacts 23a to the negative side of source i9. The instant that this circuit is completed, the capacitors 3l and 32 which are connected in parallel with resistors 21, 28, 23 and 33 immediately draw a large charging current from the excitation source and this charging current iiows through the field winding lac. rlhis charging current rises practically instantaneously to its maximum value and thereby produces a vigorous and effective forcing action on the excitation of dynamo-electric machine I8. The magnitude of this forcing action is so great as to overpower the retarding action of the capacitor 23 on the rate of rise of the voltage of dynamoelectric machine i8 and consequently the voltage of the machine rises rapidly and applies a vigorous and eective forcing action'to the main separately excited field llc of supply generator I2. As a result of energization of the field winding 12k, generator I2 begins to supply current to the swing motors IU and II. The current supplied to these motors rises rapidly to a maximum value such as represented by the point 31a on the curve 31 in Fig. 3. As a result the motors IU and i! begin to rotate and swing the dipper towards the right.

When the voltage across the resistors 21, 23, 23 and 30 has risen to full value, the capacitors 3l and 32 become fully charged so that the charglng current ceases and the forcing action produced by these capacitors likewise ceases.

The master switch is next moved to the second position in which the contacts 20c are closed to short circuit the resistor 30. As a result of this operation, the excitation of the control field winding |80 and the excitation of the separately excited field winding I2k are further increased and the operation is now in accordance with curve 38 of Fig. 3. The current to the motors I0 and II will quickly rise to a maximum value uch as represented by the point 38a on curve 38. s On the fourth point of the master switch, the` vthat during the swing of the cam actuated contacts 20d are closed to shcst circuit the resistor section 25 thereby further increasing the excitation of the control field winding I8c and the separately excited field winding I21c with the result that the operation on this point of the master switch is now in accordance with the volt-ampere characteristic represented by the curve 39 of Fig. 3.

On the fifth point of the master switch, the cam actuated contacts 20e are closed to short circuit resistor section 28 and further operation is in an cordance with the volt-ampere characteristic represented by the curve 25.

Since the forcing capacitors 3! and 32 were fully charged when the master switch was being moved through the second, third, fourth and fifth positions, the capacitors produced no forcing action and consequently the anti-hunt capacitor 23 was effective during this stageof the operation to limit the rate of rise of voltage of the dynamo-electric machine I8 and therefore of the generator I2 to safe values. In other words, the forcing action of the capacitors 3| and 32 is ei fective only for a brief period following the completicn of the energizing circuit for the control iield winding of the dynamo-electric machine I 8 when it is most needed, and after this initial forcing action, the anti-hunt capacitor 23 is efiective to limit the rate of rise of excitation of the generator field winding I2k.

As a result of the foregoing operation, the speed of the motors I0 and II rise rapidly to theirmaximum values and the dipper is swung rapidly toward the right. Although the foregoing operations were described as taking place in a sequence comprising a series of i steps, it will be understood that in practice, the operator may move the master switch 20 so rapidly through its successive operating positions that the successive steps take place without any substantial intervening time interval. In other words, the successive steps are merged into a ccntinuous operation. It will he noted that when the contacts 20e close to short circuit the resistor section 28, the forcing capacitor 32 is also short circuited and therefore completely discharged.

Since the entire mass of the cab and the electrical apparatus mounted therein is in motion during the swing motion, the inertia forces involved in acceleration and deceleration are enormous. Therefore, it becomes necessary to apply a vigorous and effective braking action to the apparatus as the dipper approaches a position over the loading car. It may be assumed dipper to the right, motor I 0 is in such the current supplied to the a direction that the polarity of the left-hand terminal oi' the resistor I1 is positive and the polarity of the right-hand terminal of this resistor is negative. Consequently, as long as the current supplied from the generator I2 to the motor I 0 is less than the values at which the voltage drop across the resistor I1 becomes equal to the voltage drop between themidtap of the resistor I l and the rectifier 22, no current ows in the field winding I3h. When the current supplied to the motor Il) has such a value that the voltage drop across the resistor exceeds the bias voltage between the midpoint of resistor I4 and the rectiiier 22, current ows from the left-hand terminal oi' resistor I 1 through the field winding Hh, through the right-hand side of resistor I4 and rectifier 22 to the right-hand terminal of resistor I1 with the result that the field winding Ilh opposes the excitation of the main control field winding I8c. ing action necessary to overcome the inertia of the swing motion and to stop the dipper as it swings over the loading car, the drive motors I0 and II are plugged by moving the master switch rapidly from the fifth right-hand position back through the of! position and then over to the fifth left-hand position. As the master switch is moved back through the fourth, third, second and first positions successively, the resistors 28, 28 and 30 are reinserted in the circuit of the control field winding |80 thereby decreasing its energization and likewise decreasing the energization of the separately excited field winding I2k of generator I2. The reinsertion of resistors 28, 29 and 3|) causes the forcing capacitor 32 to become charged again and since the forcing capacitor 3| wasnever discharged, both capacitors 3| and 32 are charged before the master switch reaches the neutral position.

As the excitation of the control field winding Ic is decreased, the voltage of the supply generator I2 is correspondingly decreased. Owing to the inertia of the apparatus involved in the swing motion and in the driving motors I0 and II, the

motors I8 and II operate as generators and pump current back into the generator I2 which now operates as a motor. This pump back current is in the opposite direction from that of the current which was supplied by the generator to the motors I0A and II when they were operating as motors. Consequently, the voltage drop across the resistor I1 is reversed and if the voltage drop across the resistor exceeds the bias voltage between the midpoint of resistor Iland rectifier 2|, current will iiow 'from the right-hand terminal of resistor I1 through rectifier 2|, through field windings IBh in a direction to aid the magnetization of control field winding I8c and then to the left-hand terminal of resistor I1. In other words, the current limit field winding Ih tends to maintain the voltage of the generator I2 at such a value that the operation will continue to be in accordance with the volt-ampere characteristic represented by the curve 25 during this pump back period thereby tending to avoid any unde-` sirable current peaks. At thel same time, the anti-hunting capacitor 23 discharges through the separately excited field winding I2k as the voltage of the dynamo-electric machine I8 VVdecays toward zero thereby augmenting the action of the field windings I8h in maintaining the voltage of the generator I2 at such a value that undesirable current peaks are, avoided. The pump back current from the motors I0 and II to the generator i2 produces an effective regenerative braking action and causes the speed of the motors I0 and II to decrease rapidly.

In the neutral position of the master switch 28 the control ileld'windingl I8c is deenergized. As the master switch 2li is advanced through the first, second, third, fourth and fifth positions on the left-hand side the excitation of the reversecontrol field winding I8d is increased from zero to maximum in a manner identical with that- 65 described for the field winding I8c with the exception that the forcing capacitors 3| and 32 do not produce any forcing action owing to the fact that they are completely charged before the master switch reaches the neutral position and the transition from the first position on the right-hand side of the master switch to the first position on the left-hand side of the master switch is usually much less than the time required for the capacitors to discharge. As the master In order to produce the brak- I .of the motors I0 and II.

switch is advanced through the first, second, third, fourth and fifth positions on the left-hand side the excitation of the separately excited field winding I2k is reversed and this causes the voltage of the generator I2 to reverse. Owing to the large inertia of the apparatus, the speeds of the motors I8 and II do not reach zero when the master switch 28 is moved to the neutral position and consequently, when the voltage'of the generator I2 is reversed the voltage is added to the voltage of the motors I8 and Il. This action is known as pluggingr the motors and it is a very effective method of reducing the speed of the motors I0 and iI to zero because the current lflowing in the loop circuits is increased by the addition of the generator voltage to the voltages During this plugging period, the forcing capacitors 3| and 32 do not force the field I8b since they were fully charged before the master switch reached the neutral position and consequently the anti-hunt capacitor 23 is effective in causing a current to flow through the anti-hunt field winding |81', in response to a change of the voltage of the dynamoelectric machine I8 in such a direction as to oppose the change. Thus, during the pump back and plugging periods the current limit field winding I8h and the anti-hunt field winding serve to control the rate of change of excitation of the separately excited field winding |21: of generator I2 in such a manner thatA undesirable current peaks in the armature loop circuits are avoided during reversal.

As the speeds of the motors 18 and II reach zero owing to the pump back and plugging operations described in the foregoing, the dipper stops v above the loading car and is dumped.

Since the master switch is not in the ofi position whenthe motors I0 and II reach zero speed,

they immediately begin to accelerate in the reverse direction to return the dipper to the digging point at the bottom of the bank. Since the forcing capacitors 3| and 32 are already fully charged, they do not draw any charging current and consequently do not produce any forcing action during this acceleration in the reverse" direction. However, forcing action is not required at this time since the voltage of the supply generator I2 has already been increased to a high value during the plugging operation. During Athis acceleration in the reverse direction, the current limit field winding I8h becomes energized when the voltage drop across resistor I'I exceeds the bias voltage between the rectifier 22 and the midpoint of resistor I4` and thereafter opposes the field winding I8d so as to limit the current supplied to the motors I0 and II in accordance with the volt-ampere characteristic represented by' the portion of the curve 25 between points b and c.

As the .dipper approaches the digging pointofthe bank, the master switch is returned to the or! position.. As explained in the foregoing, this results in weakening the excitation of the dynamo-electric machine Il and decreasing the voltage of the generator I2 so that the motors I0 and II pump back into the generator and thereby produce an effective braking action which reduces the speed of the motors. During this pump back period the field winding IBh.v and the anti-hunt capacitor 23 will lact with the neld winding Ild to maintain the excitation of the separately excited field winding I2lc at such a value that undesirable current peaks are 15 avoided. After an interval of. time determined by the setting of time delay device 26a, the contacter 26 closes its contacts to short circuit the anti-hunt capacitor 23 and complete a circuit through the anti-hunt field winding li and the resistor 23a to the load brushes of the dynamo-electric machine I8. The voltage produced by the residual magnetism of the dynamo-electric machine I8 causese current to flow in the circuit through the anti-hunt iield winding i871 in such a direction that the magnetism of the anti-hunt eld winding opposes the residual magnetism and accordingly the residual magnetism is quickly and effectively destroyed and the motors l and l l are quickly stopped.

The setting of the time delay device 26a is such that the contacts of the contactor 26 are maintained open for the period required for the master switch to he moved through the neutral position during a usual plugging operation.

As previously pointed out, the inertia forces involved in the swing motion are extremely large owing to the fact that the entire mass of the shovel must be accelerated and decelerated when the clipper is swung from one position to another. The inertia forces involved in the hoist motion, on

the contrary, are quite small owing to the fact that only the mass of the dipper and dipper-sticl; are accelerated and decelerated. Consequently, the requirements for the hoist motion are somewhat diferent from the requirements for the swing motion. In order that the maximum productivity of the shovel shall be obtained it is necessary that the dipper be lowered to the bottom of the bank at a very high speed and quickly reversed at the bottom of the bank so that the digging operation may bestarted with a minimum of time lost during the reversal operation. The apparatus shown in the modification of Fig. 2 is particularly adapted to provide the snappy action essential to maximum productivity. As indicated in Fig. 2, the hoist motor 4U is supplied from an adjustable voltage generator 4l from which the self-excited shunt eld winding is omitted and which is provided only with a separately excited field winding 41a and a relatively small series differential field winding Mb. The motor 4D is provided with a separately excited field winding 40a and with a commutating field winding 43h. Although not shown the generator 4l may also be provided with a commutating field winding.

The separately excited eld winding Ha of the generator 4I is connected to the load brushes of a dynamo-electric machine 42 which is similar to the machine i 8 of Fig. 1. The machine 42 is provided with a main control eld Winding 42a, a current limit field winding 42b and an anti-hunt field winding 42o. l

The anti-hunt iield winding 42o is connected to the secondary winding of an anti-hunt transformer 43 whose primary winding is connected to the load brushes of the dynamo-electric machine 42. The flux of the anti-hunt field Winding 42e is proportional to the rate of change of voltage of the dynamo-electric machine 42 and the connections between the anti-hunt eld winding and the secondary of the anti-hunt transformer 43 are such that the direction of the ux opposes the change in voltage.

The main control iield winding 42a of the.

dynamo-electric machine 42 is connected to a source of separate excitation represented by the supply line dit for excitation in either direction by means of a multi-position reversing type master switch t. en adiustable resistor 46 is connected in series relationship with the control field winding 42a and is varied in response to movement of the master switch to its successive positions thereby to adjust the degree of excitation of the control iield Winding 42a. The right-hand terminal of the control eld winding 42a is connected to the midtap of a resistor 41 which is connected across the excitation source 44 and the left-hand terminal of the control field winding 42a is connected to the upper terminal of the excitation adjusting resistor 46 so that current is caused to ow through the iield winding 42a in one direction or the other as the lower terminal of the resistor 46 is connected to one side or the other of the excitation source 44 by means of the master switch 45.

The current limit field winding 42h is connected across the series differential field winding 41h of the generator and the commutating field winding 40h of the motor by means of conductors 48 and 49 so as to be responsiveto the voltage drop across these field windings. Suitable means are provided for introducing a bias voltage in the circuit of the current limit field Winding 42h so that the generator 4l will have the desired steeply drooping volt-ampere characteristic such as represented by the curve 25 of Fig. 3. These bias voltage means comprise a transformer 50 having a primary winding connected to a suitable source of alternating voltage, a full wave rectifier 5| connected across the secondary winding of the transformer and a resistor 52 connected across the output terminals of the rectifier. The conductor 49 is connected to the midpoint of the resistor 52 and reversely connected rectiiiers 53 and 54 are connected between the terminals of the resistor 52 and the current limit field winding 42h so that a bias voltage of correct polarity will be available for either direction of current ow in the loop circuit of the generator 4| and the motor 4U.

A forcing capacitor 55 is connected in parallel with the excitation adjusting resistor 46 and this capacitor performs the same function as the capacitors 3| and 32 in the modification of Fig. 1.

Owing to the fact that the clipper may strike a large rock or other snag as it cuts up through the bank, it is desirable to have a current limit for the hoist motor 40. Since a motor can commutate a higher current at low voltage than at high voltage it is desirable that the current limit shall be adjusted in inverse proportion to the applied voltage. To provide such a current limit means are provided for varying the bias voltage of the current limit field winding in inverse proportion to the voltage across the terminals of the hoist motor 40. These means comprise a saturable core type reactor 56 having its direct current saturating winding 56a connected across a resistor 5l which is connected in series with a resistor 58 across the armature terminals of the motor 40 and having its reactance winding 56h connected across the secondary winding of the transformer 50. The transformer 50 is designed so that its secondary voltage decreases rapidly with the load. In other words, the transformer has poor regulation.

When the voltage applied to the motor terminals is high, the current flowing in the saturating winding 56a of the saturable core reactor is correspondingly high with the result that the reactance of winding 56h is low. This increases the load on the transformer with the result that the secondary voltage is correspondingly decreased. Conversely, for low voltages across the motor 40. the current supplied to the saturating i linearly in inverse asonese winding 58a is decreased and the reactance of the reactance Winding 56h is correspondingly increased, This decreases the load on the secondary winding of the transformer 50 with' the result that the bias voltage supplied to the circuit of the current limit field winding 42h is increased. In other words, the bias voltage is caused to be inversely proportional to the voltage across the armature terminals of the hoist motor 40. The result of varying the bias voltage in inverse proportion to the voltage existing across the armature terminals of the hoist motor is that at high voltages the current limit field winding 42h becomes energized at lower values of motor armature current than at low voltages. In other words, the current limiting action of the current limit field winding |21) takes effect at current values that are inversely proportional to the motor armature Voltage. This produces a very effective power limit for the hoist motor ,with the result that any substantial injury to the motor or shovel apparatus is averted in the event that the dipper should strike a massive obstacle in the bank while digging at high speed.

i The amount of current which a motor can commutate is not linearly inversely proportional to the applied voltage. Therefore, the motor can be caused to perform the maximum amount of "work hich it is capable of performing by varying the current limit in accordance with the commutating ability of the motor. For example, very satisfactory perfomance is Aobtained by varying the current limit only slightly as the voltage varies from zero voltage to half voltage and by varying the current limit at a substantially greater rate and in inverse proportion to the voltage between half voltage and full voltage. In other words, the motor can be worked close to its maximum commutating limits by nonlinear adjustment of the current limit in inverse proportion to the applied voltage. Such non--` linear characteristic may be produced by means of a resistance 59 having a non-linear volt ampere characteristic. This resistance may be made of any material having a non-linear characteristic such as copper oxide` It is preferred, however, to use a resistance material consisting of a composition of silicon carbide crystals held together by a suitable binder. Resistance material of this kind is described in United States Patent 1,822,742, Karl B. McEachron, dated September 8, 1931. The current iiowing through invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

- 1. A control system comprising in combination an adjustable voltage generator provided with a self-excited shunt field winding and with a main separately excited field winding, an electric motor supplied from said generator, means for supplying current to said separately excited iield winding comprising an armature reaction excited dynamo-electric machine having its load brushes connected to said separately excited iield4 winding and having a control eld winding, means for controlling said generator to have a relatively steeply drooping volt-ampere characteristic comprising a field winding on said dynamo-electric machine energized by the current exchanged between said generator and motor so as to oppose the magnetization of said control eld winding, and means for varying the excitation of said control iield winding thereby to adjust the maximum value of the current supplied to said motor. Y

2. A control system comprising in combination an adjustable voltage generator having a main separately excited field winding and a selfexcitedshunt field winding, an electric motor supplied from said generator, means for supplying said 'main field windingic'omprising an arvmature reaction excited dynamo-electric maa resistance of this character increases approxi- I mately with the cube of the impressed voltage.

The resistance 59 is preferably connected in the current limit adjusting circuit, i. e., in circuit with the saturating winding 58a of the saturable core reactor 56. As a result, the current limit of the supply generator is adjusted nonproportion to the voltage` The remaining features of the operation of the apparatus of Fig. 2 are so similar to th'ecorresponding features of operation of the system of Fig. 1 that further description is omitted.

Although inaccordance with the provisions of the patent statutes this invention is described as embodied in concrete form and the principle thereof has been described together with the best mode in which it is now contemplated applying that principle, it will be understood that the apparatus shown and described is merely illustrative and that the invention is-not limited thereto,

'since alterations and modiiications will readily suggest themselves to persons skilled in the art without departing from the true spirit o! t chine having its load brushes. connected to said separately excited eld winding and having a control field winding, means for limiting tn e maximum current output of said generator cornprising an auxiliary winding on said dynamoelectric machine connected for energization by the current exchanged between said generator and motor to oppose said control field Winding, means for rendering said current limiting means ineffective when the current exchanged between said generator and motor is less than a predetermined value, and means for adjusting the current supplied to said control winding thereby to adjust the maximum value of the current supplied to said motor.

3. A control system comprising in combination an adjustable voltage generator having a main separately excited field winding. an electric motor supplied from said generator, means for supplying current to said main field winding comprising an armature reaction excited dynamo-electric machine having its load brushes connected to said main eld winding and having control tield vwinding and a winding for compensating the armature reaction of the load currentof said machine, means for limiting the output current of said generator comprising a iield winding on said machine opposing said control neld winding and excited by the current exchanged between said generator and motor, a shunt connection for said compensating neld to cause said dynamo-electric to have a drooping volt ampere characteristic. and means for adjusting the excita.- tion of said control field winding thereby correspondingly to adjust maximum values of the current supplied to said motor.

4. A control system comprising in combination an adjustable voltage generator provided with a. mainseparately excited iield winding, an electric motor supplied from said generator, means for supplying current to said separately excited field winding comprising an armature reaction excited dynamo-electric machine having its load brushes connected to said generator .field winding and having a. control field Winding, a reversing master switch for controlling the excitation of said control field winding thereby to control the direction of rotation of said motor, and means comprising an auxiliary field winding connected in a loop circuit with the armature of said dynamo-electric machine to oppose said control field winding and a capacitor connected in said loop circuit for causing current to be conducted through said opposing neld winding when the voltage of said dynamo-electric machine is changing in response to reversing operation of said master switch thereby to limit the peaks of current' exchanged between said generator and said motor during reversal.

5. A control system comprising in combination an adjustable voltage generator provided with a separately excited field winding, an electric motor supplied from said generator, means for supplying exciting current to said field winding comprising an armature reaction excited dynamoelectric machine having its load brushes connected to said field winding and having a control field winding, a reversing master switch having an ofi position and selectively operable from said position to forward and reverse operating positions to control the excitation and polarity of said control field winding thereby to control the direction of rotation of said motor, means comprising an auxiliary field winding connected in a loop circuit with the armature of said dynamo-electric I machine to oppose said control field winding and a capacitor included in said loop circuit for causing current to be conducted through said auxiliary field winding when the voltage of said dynarno-electric machine is changing in response to reversing operation of said master switch thereby to limit the peak values of current exchanged between said generator and motor during reversal, and a contacter responsive to operation of said master switch to said ofi position for short circuiting said capacitor thereby to provide a circuit through said opposing eld winding to destroy the residual magnetism of said dynamo-electric machine and rapidly reducethe current in the separately excited field winding of said generator.

6. A control system comprising in combination an adjustable voltage generator provided with a separately excited field winding, an electric motor supplied from said generator, means for supplying exciting current to said field winding comprising an armature reaction excited dynamoelectric machine having a control field winding and having its load brushes connected to said separately excited field winding, an opposing I field winding on said dynamo-electric machine energized by the current exchanged between said generator and motor, thereby to cause said generator to have a relatively steeply drooping voltampere characteristic, means for controlling the direction o1" rotation of said motor and for adjusting the maximum value of the current supplied by said generator comprising an adjustable resistor in circuit with said control field winding and a multi-position reversing master switch for adjusting said resistor and controlling the direction of the current supplied to said control eld winding, means providing field forcing of said generator when starting said motor from rest comprising a capacitor connected in parallel with said resistor, and means for limiting the peak values of current exchanged between said generator and said motor during plugging operation comprising a second opposing field winding on said dynamo-electric machine connected in a loop circuit with the armature thereof and a capacitor included in said loop circuit.

7. A control system comprising in combination, an adjustable voltage generator provided with a separately excited field winding, an electric motor supplied from said generator, means for supplying exciting current to said separately excited field winding comprising an armature reaction excited dynamo-electric machine having a control field winding and having its load brushes connected to said separately excited field winding, means for controlling the volt-ampere characteristic of said generator Comprising an opposing field winding on said dynamo-electric machine excited by the current exchanged between said motor and generator, means for rendering said volt-ampere characteristic control means inactive when the current flowing between said generator and motor is less than a predetermined value, and means responsive to the voltage supplied to said motor for adjusting the value atA which said control means is rendered inactive.

8. A motor control system comprising in combination, an adjustable voltage generator provided with a separately excited field winding, an electric motor supplied from said generator, means for supplying exciting current to said field winding comprising an armature reaction excited dynamo-electric machine 4provided with a. control field winding, means for controlling said generator to have a volt-ampere characteristic such that a relatively small change in current is accompanied by a relatively large change in voltage comprising an opposing field winding on said dynamo-electric machine excited by the current flowing between said generator and motor and means for supplying an opposing biasing voltage to the circuit of said opposing field winding so that said opposing field winding remains deenergized for values of said current less than a predetermined value, and means responsive to the voltage supplied to said motor for varying said biasing voltage in inverse proportion comprising a source of alternating voltage, a saturable core type reactor having its saturating Winding energized by the voltage supplied to said motor and having its reactance winding connected in circuit between said source and said biasing voltage supply means,

9. A motor control system comprising in combination, an adjustable voltage generator provided with a separately excited field winding, an electric motor supplied from said generator, means for supplying exciting current to said field winding comprising an armature reaction excited dynamo-electric machine provided with a control field winding, means for controlling said generator to have a volt-ampere characteristic such that a relatively small change in current is accompanied by a relatively large change ln voltage comprising an opposing field winding on said dynamo-electric machine excited by the current flowing between said generator and motor and means for supplying an opposing biasing voltage to the circuit of said opposing field winding so that said opposing field winding remains deenergized for values of said current less than a predetermined value, comprising a resistor connected in the circuit of said opposing field winding, a transformer and a rectifier in the secondary circuit thereof for supplying a rectified voltage to said resistor, and means for varying said biasing voltage in inverse proportion to the voltage supplied to said motor comprising a saturable core type reactor having its reactance winding chine excited by the a plied to, saiamotor' frconnected in said secondary circuit and havingv its saturating winding connected to be energized by the voltage supplied to said motor.

10. A control system comprising in combination, an adjustable voltage generator provided with a separately excited eld winding, an electric motor supplied from said generator, means for supplying exciting current to said separately g excited eld winding comprising an armature reaction excited dynamo-electric machine having to the circuit of said opposing eld winding so that said opposing neld winding remains deenergized for values of said current less than a predetermined value, and means responsive to the voltage supplied to said motor for varying said biasing voltage in non-linear inverse proportion' to said voltage comprising electrical connections including a non-linear resistance from said motor to said bias voltage supply means.

a control field winding and having its load brushes ing, means for controlling the volt-ampere characteristic of said generator comprising an' opposing eld winding on said' dynamoelectric. ma-

Acurrent exchanged between Aconnected to said separately excited -eld windsaid motor and generator,gmeans for said volt-ampere characteristic control meenam active when the current'owing: *said generatorA and. motor is iess'tlian atl-predetermined' 12. A motor control system comprising in combination, an adjustable voltage generator pro-A vided with 'a separately excited fieidrvwinding,

an electric motor supplied Vfrom/said -gerterilr.or,v

means for supplying exciting correntto-said field dynamo-electrlnachine provided with a control means for controlling said 'generai'tor to- Barea voltea-'repare characteristic such value, and means-responsive to thevvoltage sul adjusting the value at which-santi control rendered inactive inv 'proportion w said voltage.

IIe-Lentate; control system comprising in com@v mation, adjustable voltage generator provided witha separately windingsuppliedv from said contrai-idd for controlling saidV generator'ftdhavea volt-ampere characteristic such'thatarelativelysmall changeincurrentis accompanied by a relatively large change in volt-- age comprising an opposing field winding on said dynamo-electric machine excited by the current biasingvoltage flowingv hetweeni'said generator means tor'sumalyingv an opposing companied by a relatively large that a relatively small change in current is acchange in voltage comprising an' opposing field winding on said dynamo-electric machine excitediby the current Iiowing between said generator-and motor and means for supplying an opposing-.biasing voltage Vtothe-circuit of'sald @opening eldwinding so that saidopponffield winding) remains deenergiled for'values of saidcurrent less than a predetermined-value, andmeansresponsiveto the 'memppiieewmo motor-'ferme said ai man;

voltage in' nonlinear inverse proportion amprisinga souroeof alternating voltige. a-

saturable'coretypereactorhavingits saturatingA winding energized bra voltage derived from' said motor and its reactanee winding connected in circuit between said source and said biasing voltage supply means, and s. non-linear resi'stttllce ineluded in circuit with said sa'tnrating windingi r 

